In wood-based panels, plywood has good physical and mechanical properties, beautiful and natural wood texture, which is unmatched by other wood-based panels. However, with the decreasing of the world's forest resources, the large-diameter logs necessary for the production of plywood are becoming increasingly scarce, and the use of fast-growing small-diameter materials has become more and more common. The production of large-format veneers with small-diameter materials has a low yield. Therefore, in the production, plywood is produced by a single-plate length. There are four main types of board extension: docking, lap, miter and finger joint. The latter two methods are allowed in the plywood national standard, and a variety of equipment are currently available. However, the corresponding processes and special equipment must be added. In actual production, most small factories and even some large enterprises still use the docking or lap joint process to produce.
The purpose of this study is to investigate the effect of the docking and lap jointing of veneers on the performance of thick plywood, and provide a theoretical basis for the production and application of plywood.
1 Materials and methods
1 1 Test materials
Veneer: Yi Yang, thickness 1 5mm
Adhesive: urea-formaldehyde resin, solid content 52% ~ 56%
The above materials were taken from the pilot plant of the school.
1 2 Test equipment
1 0 0T experimental hot press
MW -4 wood universal ability testing machine
1 3 Test methods
1 3 1 lengthening method
The long core board is docked and overlapped. The lap joints are of two forms: (1) lap joints and then lap joints; (2) lap joints directly. Each method is respectively pressed into three groups of 5 layers, 7 layers and 9 layers, and the joint portions of the two core layers or more are staggered.
1 3 2 hot pressing process
Glue amount: 2 80g/m2 (double-sided)
Hot pressing temperature: 1 1 5 °C
Hot pressing pressure: 10 MPa
Hot pressing time: 1min/mm
1 3 3 Performance testing
The pressed plywood was examined for its thickness deviation, 24 h water absorption thickness expansion ratio, static bending strength, and flexural modulus.
2 Results and analysis
2 1 Effect of lap joint on sheet thickness
The effect of the two overlapping methods on the thickness of the sheet is shown in Fig. 1 and Fig. 2. It can be seen from Fig. 1 that the overlap has a certain influence on the thickness deviation of the sheet, and the thickness deviation gradually decreases as the number of layers increases. This is because the overlap is equivalent to the core, which inevitably increases the thickness of the place. Due to the misalignment of the multi-layer long core plate overlap, there is only one lap joint in each thickness direction. Therefore, as the number of layers increases, the proportion of the lap joint decreases, and the influence on the thickness is also Gradually weakening the impact of the overlap on the water absorption thickness expansion rate is large, as shown in Figure 2. On the one hand, the slab compression ratio of the lap joint is large. On the other hand, the splicing of the slab at the lap joint is not tightly combined with the adjacent layer veneer, and the gap at the end of the veneer is easy to enter. In particular, the overlap is more pronounced when the blank is formed. For the lapped panel, the lap joint is basically flattened, some of the fibers are crushed, and the combination is better when the sheet is hot pressed, and the water absorbing thickness expansion ratio is close to that of the ordinary board. Similar to the effect on thickness deviation, the number of layers increases and the effect is weakened. In the case of nine layers, the plates produced by the two overlapping methods are very close to the ordinary plates.
2 2 Influence of the length of the single board on the mechanical properties of the board
2 2 1 Influence of lap joint on mechanical properties of sheet
The effect of lap joint on the static bending strength and flexural modulus of the sheet
3. Figure 4.
It can be seen from Fig. 3 and Fig. 4 that the lap joint length of the single board has little effect on the mechanical properties of the sheet. The two-layer veneer at the joint actually acts as a reinforcement to offset the effect of the joint. However, in the lap joint production, it should be noted that the joint should be coated with adhesive to ensure a firm glue.
2 2 2 Influence of butt joint on mechanical properties of sheet
The effect of docking on the static bending strength is shown in Fig. 5 and Fig. 5, and the docking has a great influence on the static bending strength. And the greater the decrease as the number of layers increases. The number of layers ranges from 5 to 9, and the static bending strength decreases from 18 2 % to 2 5 6%. The tensile strength of the long core plate at the butt joint is zero, which weakens the ability to resist stress. The effect of docking against the flexural modulus is small. Under the test conditions, since the pressure used in the test is small, the pressure is mainly absorbed by the watch plate, so the measurement results are very close.
3 Conclusion
In summary, the effects of the two lengthening methods on the performance of thick plywood vary. The docking method has a great influence on the mechanical properties, while the lap joint mainly affects the thickness deviation. For plywood of different purposes, as long as one of the two methods is properly used, the process can be simplified and the cost can be reduced without affecting the use effect. If it is used as a cement formwork, it can be lapped, and when it is used as a non-structural material such as partition wall or furniture, it can be docked.
The purpose of this study is to investigate the effect of the docking and lap jointing of veneers on the performance of thick plywood, and provide a theoretical basis for the production and application of plywood.
1 Materials and methods
1 1 Test materials
Veneer: Yi Yang, thickness 1 5mm
Adhesive: urea-formaldehyde resin, solid content 52% ~ 56%
The above materials were taken from the pilot plant of the school.
1 2 Test equipment
1 0 0T experimental hot press
MW -4 wood universal ability testing machine
1 3 Test methods
1 3 1 lengthening method
The long core board is docked and overlapped. The lap joints are of two forms: (1) lap joints and then lap joints; (2) lap joints directly. Each method is respectively pressed into three groups of 5 layers, 7 layers and 9 layers, and the joint portions of the two core layers or more are staggered.
1 3 2 hot pressing process
Glue amount: 2 80g/m2 (double-sided)
Hot pressing temperature: 1 1 5 °C
Hot pressing pressure: 10 MPa
Hot pressing time: 1min/mm
1 3 3 Performance testing
The pressed plywood was examined for its thickness deviation, 24 h water absorption thickness expansion ratio, static bending strength, and flexural modulus.
2 Results and analysis
2 1 Effect of lap joint on sheet thickness
The effect of the two overlapping methods on the thickness of the sheet is shown in Fig. 1 and Fig. 2. It can be seen from Fig. 1 that the overlap has a certain influence on the thickness deviation of the sheet, and the thickness deviation gradually decreases as the number of layers increases. This is because the overlap is equivalent to the core, which inevitably increases the thickness of the place. Due to the misalignment of the multi-layer long core plate overlap, there is only one lap joint in each thickness direction. Therefore, as the number of layers increases, the proportion of the lap joint decreases, and the influence on the thickness is also Gradually weakening the impact of the overlap on the water absorption thickness expansion rate is large, as shown in Figure 2. On the one hand, the slab compression ratio of the lap joint is large. On the other hand, the splicing of the slab at the lap joint is not tightly combined with the adjacent layer veneer, and the gap at the end of the veneer is easy to enter. In particular, the overlap is more pronounced when the blank is formed. For the lapped panel, the lap joint is basically flattened, some of the fibers are crushed, and the combination is better when the sheet is hot pressed, and the water absorbing thickness expansion ratio is close to that of the ordinary board. Similar to the effect on thickness deviation, the number of layers increases and the effect is weakened. In the case of nine layers, the plates produced by the two overlapping methods are very close to the ordinary plates.
2 2 Influence of the length of the single board on the mechanical properties of the board
2 2 1 Influence of lap joint on mechanical properties of sheet
The effect of lap joint on the static bending strength and flexural modulus of the sheet
3. Figure 4.
It can be seen from Fig. 3 and Fig. 4 that the lap joint length of the single board has little effect on the mechanical properties of the sheet. The two-layer veneer at the joint actually acts as a reinforcement to offset the effect of the joint. However, in the lap joint production, it should be noted that the joint should be coated with adhesive to ensure a firm glue.
2 2 2 Influence of butt joint on mechanical properties of sheet
The effect of docking on the static bending strength is shown in Fig. 5 and Fig. 5, and the docking has a great influence on the static bending strength. And the greater the decrease as the number of layers increases. The number of layers ranges from 5 to 9, and the static bending strength decreases from 18 2 % to 2 5 6%. The tensile strength of the long core plate at the butt joint is zero, which weakens the ability to resist stress. The effect of docking against the flexural modulus is small. Under the test conditions, since the pressure used in the test is small, the pressure is mainly absorbed by the watch plate, so the measurement results are very close.
3 Conclusion
In summary, the effects of the two lengthening methods on the performance of thick plywood vary. The docking method has a great influence on the mechanical properties, while the lap joint mainly affects the thickness deviation. For plywood of different purposes, as long as one of the two methods is properly used, the process can be simplified and the cost can be reduced without affecting the use effect. If it is used as a cement formwork, it can be lapped, and when it is used as a non-structural material such as partition wall or furniture, it can be docked.
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